In the days of using pot-bellied stoves to heat homes, simple round ductwork was used to convey the noxious fumes and smoke out of house. The house itself was leaky enough to bring in fresh outdoor air. Such early ducts were hand-made, often out of tin, which is less corrosive than steel, hence the early duct fabricators were called tin-knockers.
Today, with the advent of galvanization, most general-purpose building ducts are made of galvanized steel “sheet metal,” so called because it is produced in thin sheets of standardized gauges. Specialty ducts are made of various materials such as fiberglass, aluminum, black iron, stainless steel, etc., but the installers are still called tin knockers or tinners.
With today's tight building structures, central heating and cooling systems supply air to and exhaust air from multiple rooms on multiple levels. Ducts are heavily engineered systems, used to supply fresh outdoor air to the building interiors, to exhaust fumes and odors to the outside, and to distribute comfort air to all occupants. Specialty systems such as toilet exhaust, laundry exhaust, or fume hood exhaust are engineered and built separately from the general building exhaust and supply. Duct design has become an engineering specialty.
To reach all areas of he building, ductwork must be constructed with numerous bends and offsets changing direction, elevation, or size or any combination thereof.
Because different volumes of air are delivered to different spaces, the size of the ductwork changes in proportion to the volume of air delivered to or exhausted from various spaces. The piece of ductwork between ductwork runs of different sizes or different directions is called a change fitting.
In supply systems generally, central fans emit conditioned air at a given velocity and pressure and as the length of the ductwork increases, the static pressure of the air drops. Fans must be selected to provide sufficient pressure to force the air stream to the end of the duct system. In addition, every bend or change in the duct size further increases the static pressure requirement of the fan. Fans must be sized large enough to deliver air to the farthest reach of the ductwork system. Exhaust systems are sized similarly, with the ducts increasing in size as they approach the exhaust fan.
A ductwork design engineer must attempt to use the smallest duct possible to keep the cost of material low. However, he or she must avoid frequent changes in duct sizes to keep the cost of the duct fabrication and installation low. At the same time, the engineer must size the fan to the minimum horsepower possible to keep the initial cost of the equipment and the operating costs of the equipment at a minimum. Larger equipment also utilizes valuable floor space in equipment rooms. Equipment rooms are not rentable space, so building owners would prefer to allot as little space to mechanical equipment as possible.
Traditionally, “radius elbows” and “radius offsets” are believed to cause a much lower drop in static pressure than “square throat elbows” and “angled offsets.” Design engineers will typically insist on the installation of radius fittings to reduce the system pressure drop and thus allow the installation of smaller equipment.
However, despite computerized design, plasma cutting, and automated bending, the final duct shape is still largely constructed in traditional fashion and installed on the job site by hand. Constructing and installing the radius fittings is the most time-consuming aspect in the ductwork fabrication and installation process, and hence one of the most costly.
Further, despite the computerized optimization of the cutting layout of the fittings from the flat stock sheet metal, traditional radius fittings of all types still result in unacceptable wasted flat stock. The waste material is scrap metal.
The Scrapless Fabrication method or process of manufacturing or fabricating ductwork offsets, elbows, tees, wyes, transitions and change fittings from sheet metal materials is a novel and inventive manner of performing the work that substantially reduces waste material and fabrication inefficiencies. This method is described in U.S. patent application Ser. No. 11/688,544, which is incorporated herein. The Scrapless Fabrication method eliminates the waste resulting from cutting fitting patterns from standard size material sheets and coils of sheet metal materials and simplifies the final hand fabrication, assembly and installation of those fittings. Despite testing results, Scrapless Fabrication fittings may not satisfy traditionally trained engineers' concerns about the pressure losses associated with non-radius fittings.
It is an object of the invention described herein to provide a “Lean Duct Manufacturing” system that substantially decreases the waste produced in the conventional radius fabrication method, substantially simplifies the hand labor required for the final fabrication and installation of ductwork fittings, and provides flow characteristics, including pressure drop, substantially equivalent to standard radius fittings. The low pressure drop of the Lean Duct Manufacturing system reduces the fan size required to distribute or exhaust air and fumes, resulting in lower initial equipment costs and reduced energy costs for the life of the system.
Buildings utilizing the Lean Duct Manufacturing system may be eligible to obtain LEED certification, because the Lean Duct Manufacturing system may be applied to certain credits.
The improved flow characteristics of the Lean Duct Manufacturing fittings lower the pressure drop, which in turn requires smaller fans that use less energy. Reduction in energy usage is applicable to both LEED NC Prerequisite EA 2—Minimum Energy Performance and LEED NC Credit EA 1—Optimize Energy Performance.
To fit field conditions, ductwork is frequently fabricated on a construction site. Fabrication of Lean Duct Manufacturing fittings produces less construction waste. The reduced waste generation is applicable to LEED NC Credits M-1 and MR-2—Divert Construction Waste from Disposal. Intent of this credit is to divert construction and demolition debris from disposal in landfills and incinerators and to redirect recovered resources back to the manufacturing process or to appropriate sites for re-use. At present, recycling small pieces of scrap sheet material from a construction site is not done. Lean Duct Manufacturing reduces the overall percentage of non-recyclable waste.
Easy on-site assembly allows Lean Duct Manufacturing fittings to be transported to the job site in knocked-down condition, resulting in reduced transportation costs from one-quarter to 1/20 of the cost of transporting traditional pre-fabricated duct, depending on the finished size of the duct.
Conventional wisdom has been that only a smooth continuously curved elbow provides an acceptable amount of resistance to air flow when changing the direction of the airflow. Yet test results unexpectedly show that the Lean Duct method yields similar resistance to change in airflow at substantially lower costs and substantially greater ease of manufacturing with almost zero waste.
Lean Duct Manufacturing changes, or re-engineers, the way ductwork and change fittings are manufactured, the way individual fittings and entire ductwork systems or products can be designed to reduce waste and increase efficiencies, and most importantly, the way of doing business.
The consequences of those changes from conventional methods are significant enhancements or improvements in both fabrication and installation or assembly efficiencies. The unprecedented reductions in waste material contributes to environmental sustainability by ultimately reducing the effect of energy usage on mining operations, sheet metal material production, sheet metal fitting fabrication, and the final product and systems installation.
The new method eliminates the arbitrary nature of the custom and practice approach to the work that results in an infinite variety of fitting length and shape types by reducing the options to a set of consistent pre-determined standards, reduces the cost of construction contributing to affordable homes and offices, and provides new job opportunities for unskilled workers.